My question is about the the reference design (VD_1000W_GAN_PSFB_XDP).
(Reference design board-1000 W, 400 V phase-shifted full-bridge current-doubler with XDP™ digital power controller and CoolGaN™ HEMT)
My question is about the part highlighted below
Is this a clamp to protect the low voltage MOSFETs? If so how does it work? and is there a procedure to design it?
Thanks
Hi Ahmed,
Apologies for the delay,
The mentioned circuit will behave as an LCD snubber circuit which does not contains any active device hence, no energy consuming component. It will reduce the rate of rise of voltage and current while switching, so to achieve the protection of circuit while reducing the losses.
For reference, pl refer to the fig(s) shown below. The given circuit has four modes of operation based on the AC inputs from the sec- side of transformer. The voltage of the split sec transformer will be imposed across the high and low side MOSFET and the sec will act like a center tap transformer, as shown below. Here due to large leakage between the primary and secondary windings of transformer (due to center tap configuration) will cause voltage drops, hence external inductance added externally to model the voltage drops caused by the leakage for the analysis.
For analysis, lets simplify one part of the secondary circuit as follows-
This circuit can be analyzed in the four modes (Considering each cycle has 4 modes of operation)-
- In Mode 1, the capacitance will be charged by the reverse recovery current until it reached to its peak value.
- In the Mode 2, the current through inductor will discharge the capacitor, since the diode will be in the reverse biased.
- In Mode 3, the current through inductor will continue to flow in the same direction and since the capacitor will be fully discharged, now the capacitor will start to charge in opposite to polarity in Mode-2.
- In the Mode-4 (Fig below),
The inductor current continues to charge the Vcs, until it reaches the Vcout, and the diode will become forward biased, finally clamping Vcs to Vout, hence dissipating remaining current to output capacitor.
These modes will continue to follow in every switching cycles.
The examples given in the app note (Link attached) will help you in design.
Thank you for your patience,
Regards
Shivam
Hi Ahmed,
Apologies for the delay,
The mentioned circuit will behave as an LCD snubber circuit which does not contains any active device hence, no energy consuming component. It will reduce the rate of rise of voltage and current while switching, so to achieve the protection of circuit while reducing the losses.
For reference, pl refer to the fig(s) shown below. The given circuit has four modes of operation based on the AC inputs from the sec- side of transformer. The voltage of the split sec transformer will be imposed across the high and low side MOSFET and the sec will act like a center tap transformer, as shown below. Here due to large leakage between the primary and secondary windings of transformer (due to center tap configuration) will cause voltage drops, hence external inductance added externally to model the voltage drops caused by the leakage for the analysis.
For analysis, lets simplify one part of the secondary circuit as follows-
This circuit can be analyzed in the four modes (Considering each cycle has 4 modes of operation)-
- In Mode 1, the capacitance will be charged by the reverse recovery current until it reached to its peak value.
- In the Mode 2, the current through inductor will discharge the capacitor, since the diode will be in the reverse biased.
- In Mode 3, the current through inductor will continue to flow in the same direction and since the capacitor will be fully discharged, now the capacitor will start to charge in opposite to polarity in Mode-2.
- In the Mode-4 (Fig below),
The inductor current continues to charge the Vcs, until it reaches the Vcout, and the diode will become forward biased, finally clamping Vcs to Vout, hence dissipating remaining current to output capacitor.
These modes will continue to follow in every switching cycles.
The examples given in the app note (Link attached) will help you in design.
Thank you for your patience,
Regards
Shivam
